Cellular code-division multiple access (CDMA) radio systems, wherein mobile stations operate within “cells” whose outlines are determined by base transceiver stations (BTS's), are limited by interference. It is thus highly desirable to reduce interference. To limit overall interference, power control is employed in which mobile stations and base stations instruct each other to raise or lower their transmitting power so that sufficient, but not excessive, power is employed on forward and reverse links.
Depending on a mobile station's position within a cell and on the terrestrial features in the vicinity, a mobile station may be in communication with one or more BTS's. In FIG. 1, Mobile station 60 is assumed to be in communication with BTS's 40 and 50 both, over forward links 46 and 56 respectively, and over reverse links 47 and 57 respectively.
Forward link power control has two aspects: an open loop aspect and a closed loop aspect. The open loop aspect is used to compensate for the effect of distance variations and shadowing which are usually reciprocal on both links. The multipath fading conditions, however, are independent on the forward and the reverse links, and so the mobile station has to tell the base station how to adjust its power to compensate for fading. When the received energy-per-bit over noise density (Eb/N0) from the base station is high (or low), the mobile station sends a power control command (PC) every power control group (slot) asking the base station to decrease (or increase) its transmitted power. The power control command is sent at a fixed rate (TP). Typical rates in third generation (3G) CDMA systems are 800 or 1600 power control commands per second.
A power control command is currently sent once during each “power control group” (A power control group is also referred to as a “slot”). The power control command may be a single bit (one state connoting increase and the other connoting decrease) or it may be a multibit command to allow changing the power by different steps.
FIG. 1 depicts portions of a conventional CDMA network. The CDMA network is a portion of the public switched telephone network (PSTN) 10. Connected to the PSTN 20 is a mobile telephone switching office (MTSO) 20. Connected to MTSO 20 is base station controller (BSC) 30, which in turn connects to BTS 40 and BTS 50. BTS's 40 and 50 are equipped with antennas 45 and 55 respectively, which transmit and receive radio signals in an area located between them and generally comprising a cell.
Handoff procedures are employed for passing a mobile user from one base station to another. A mobile station is said to undergo a hard handoff if it disconnects its transmission with the current base station and initiates transmission with a new base station. If the mobile station can communicate simultaneously with two or more base stations it is said to be in soft handoff. Soft handoff is one of the advantages that CDMA cellular systems have over other cellular systems like frequency-division multiple access (FOMA). Mobile station 60 of FIG. 1 is in soft handoff, since it is in communication with two BTS's (40 and 50).
Typically, a mobile station roaming in a cell can be in soft handoff a large percentage of the time. During soft handoff the mobile station is receiving from two or more base stations. These base stations combine the signal received from the mobile station to decode the data. The mobile station also combines the signals received from the different base stations and accordingly issues a single power control command that all the base stations should follow.
In the conventional scheme, the mobile station sends power control commands at the same rate (i.e., one command during each power control group) when it communicates with a single base station as it does when it is in soft handoff, as depicted in FIG. 2. This is known as fast power control, and is useful for tracking the changes in the communication channel due to multipath fading. A mobile station enters soft handoff when the signals received from at least two base stations are within a certain strength margin and thus gain is achievable by combining these signals (diversity gain). If the power levels of the signals are significantly different, the gain from diversity is lost. Also, if one base station starts transmitting at a high power level compared to the power level of the other base station, interference is increased which reduces the system capacity. The base stations will keep transmitting at comparable power levels if the power control commands transmitted by the mobile station can be received without error.
In practice, however, error-free reception of the power control commands is unusual. The power commands are sent uncoded to avoid the delay associated with coding; hence, even when there is a good connection between the mobile station and the BTS, the error is about 5%. If the connection is weaker, the error rate can be significantly higher. Erroneous reception of the power control commands at the base stations can result in their transmitting at markedly different power levels. This results in a loss in the diversity gain that soft handoff could provide. It further can result in increasing the interference (“noise floor”) perceived by other mobile stations which reduces the overall capacity of the CDMA system.
In conventional systems, the mobile station uses the same method to issue the power control commands when it is communicating with a single base station (i.e., not in soft handoff) and when it is communicating with more than one base station (i.e., in soft handoff). However, the two situations should be dealt with differently. When the mobile sends a single power control command to the several base stations, one or more of the base stations may receive this command in error. This results in that base station transmitting at a different power level than the other base stations. This results in a loss of diversity gain that we get from soft handoff and can also increases the interference in the system, which reduces the capacity. There is thus a need to improve the reliability of transmission of power control commands from a mobile station involved in soft handoff.
It is thus an object of the present invention to provide a system for improving the reliability of transmission of power control commands from a mobile station to the base stations with which it is involved in a soft handoff.
It is a further object of the present invention to improve diversity gain in CDMA systems.
It is a further object of the present invention to reduce interference and thus increase overall capacity of CDMA systems.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.